This invention relates to electrical power distribution systems, and particularly to diagnostic tests for assessing insulation deterioration in underground electrical power distribution cables.
Underground power distribution cables are used extensively to convey electrical power to diverse locations. Typical power distribution cables are characterized by a core of one or more central conductors surrounded by a first semiconducting shield layer, a polymeric insulating layer, and a second semiconducting shield layer. Concentric neutral (“CN”) wires may be helically wound around the core construction, and a polymeric jacket may be extruded over the CN wires.
The insulating layer electrically isolates the center conductor and first semiconducting shield from the second semiconducting shield. Degradation of the insulating layer may cause shorts or arcing to occur between the first and second semiconducting shields. In extreme cases, the arcs or shorting may cause a catastrophic failure of the power cable. At the very least, the arcs or shorting may further break down the insulating layer, thereby further compromising the integrity of the power cable.
One common cause of insulation degradation is the growth of structural faults within the insulation—a phenomenon known as “treeing.” Treeing generally progresses through an insulating layer under electrical stress so that, if visible, its path looks something like a tree. “Water treeing” is the deterioration of a solid dielectric material which is simultaneously exposed to moisture and an electric field. Water trees may be driven by electro-oxidation of the insulating layer, which takes place in the direction of the local electric field and in a polar amorphous region of the insulating layer. As a result of the electro-oxidation, polymer chains are broken and a “track” or “tree path” is formed. The electro-oxidation of the polymer along this track enables water molecules to collect in the track region, which promotes further electro-oxidation of the polymer at the tip of the track.
Underground power distribution cables are particularly susceptible to “water treeing” due to their underground placement. Subterranean water molecules may infiltrate a track or tree path in the insulating layer, thereby accelerating tree formation and associated degradation of the insulating layer. After sufficient growth, such water trees may lead to conductive arcing and damage to the power distribution cable.
Low frequency testing, such as the tan-delta test, may assess deterioration of the insulation of a medium or high-voltage cable to predict the remaining life of the cable. Such testing, however, requires disconnecting and isolating the cable prior to applying a test voltage from a very low frequency power source. Low frequency testing tends to be negatively regarded by utility customers whose service must be turned off during testing. Such testing is also expensive and inconvenient for utility companies as a result of the lost service, as well as the usually costly, separate high-voltage source needed for testing.
In view of the foregoing, what are needed are apparatus and methods to perform diagnosis of energized power cable without interrupting power service. Further what are needed are apparatus and methods that utilize the usual applied power-frequency voltage on the cable to detect the existence of water trees and other damage to the cable insulation. Such apparatus and methods are disclosed and claimed herein.